Catalytic combustion in general has many advantages compared to conventional gas phase combustion. The most obvious advantages are the very low emissions, high safety (normally no flame is present and the gas mixture is too lean for gas phase ignition), controllability, insensitivity to rapid pressure/flow fluctuations, wide power range and silent operation. Typical disadvantages are the requirements of complete fuel evaporation and homogenous air/fuel mixture to eliminate the risk for thermal degradation of the catalyst. Due to the fuel evaporation requirement, combustion of gaseous fuels presents fewer challenges than liquid fuel combustion and the commercial applications are increasing. However, when it comes to catalytic combustion of liquid fuels there are still few, if any, commercial applications due to the problem to achieve complete and efficient evaporation of hydrocarbon fuels without accumulation of heavy hydrocarbon residues. Another typical disadvantage is the (electrical) energy and time needed to heat the catalytic material at start-up. This particular disadvantage has so far disqualified catalytic combustors in applications where a rapid start is crucial. Using a flame for heating at start-up results in increased emissions depending on how the combustor is operated i.e. how often the burner is started during an operating cycle. Furthermore, a flame pre-heater complicates the system since it requires fuel atomization devices and a separate flame igniter. Therefore, there is a need for a fast and low-emission start-up principle for a catalytic combustor, consuming a minimum of electrical energy. Prior art electrical start-up devices have the disadvantages of consuming a lot of electrical energy and requiring long heating time. This will delay the ignition of the catalyst, which leads to emission of high levels of unburned hydrocarbons and carbon monoxide.
JP 61-134 515 describes a catalytic burner injecting a liquid fuel spray in a swirling airflow. The fuel pump necessary to inject the fuel provides a relatively high pressure, which is costly from a power consumption point of view. The pump needed for generating high pressure also increases the cost of the assembled unit. Furthermore, preheating of the inlet air in a heat exchanger is needed to obtain complete evaporation of the fuel which further increases the complexity and cost of the assembled unit. U.S. Pat. No. 5,685,156 describes a catalytic burner for e.g. a gas turbine. This burner also requires significant amounts of power to energize the fuel pump and also demands a relatively costly pump solution.
DE 100 14 092 describes a catalytic burner having preevaporation of the fuel; after the evaporation of the fuel, the fuel is mixed with preheated air, whereupon the fuel/air mixture will pass a catalytic element and combust. The burner according to DE 100 14 092 demands fuel with a high purity and a narrow boiling range, otherwise coking and/or distillation of the fuel will occur.
US 2005/0235654 describes a catalytic burner in which the fuel is evaporated by drenching of a felt like material on a bottom of an evaporator. This solution will have the same problems as the burner of DE 100 14 092.
The problem with evaporation of liquid fuels lies in the fact that the evaporator temperature must be controllable depending on the operating conditions of the burner matching the wide power range and excellent controllability of the catalytic combustion process and accumulation of heavy hydrocarbon residuals must be prevented in order to avoid coking. Furthermore, the evaporator must reach a suitable temperature in short time during start-up in order to obtain a fast and efficient start-up process improving performance and minimizing cold start emissions. Finally, this has to be accomplished with a minimal consumption of electrical energy.